Titanium dioxide TiO2 (simply referred to as titanium oxide herein and in the claims) has hitherto been known as a substance showing a photocatalytic function. As methods for forming a titanium oxide film on a titanium metal, there have been known since the 1970s a method for forming a titanium oxide film on a titanium metal by anodic oxidation, a method for thermally forming a titanium oxide film on a titanium metal plate in an electric furnace supplied with oxygen, and a method for forming a titanium oxide film on a titanium metal by heating a titanium plate in flames of a city gas at 1,100 to 1,400° C. (see non-patent document 1). Numerous studies designed to achieve the practical use of photocatalysts have been conducted in many technical fields.
To manufacture photocatalyst products for obtaining a deodorizing, antimicrobial, anti-fogging, or antifouling effect by such a photocatalytic function, it has been common practice to impart a titanium oxide sol onto a substrate by spray coating, spin coating, or dipping, thereby forming a film. However, the resulting film is apt to peel off or wear, and thus its long-term use has been difficult.
Ultraviolet radiation with a wavelength of 400 nm or less is necessary for titanium oxide to function as a photocatalyst, but many studies have been performed on titanium oxide photocatalysts which are doped with various elements to function by visible light. For example, there is a report comparing titanium oxides doped with, for example, F, N, C, S, P and Ni, and showing the nitrogen-doped titanium oxide to be excellent as a visible light responding photocatalyst (see non-patent document 2).
As titanium oxide photocatalysts doped with other elements as shown above, proposals were made for a photocatalyst comprising a titanium compound Ti—O—X having the oxygen site of titanium oxide substituted by an atom X such as nitrogen, or an anion X, a photocatalyst comprising a titanium compound Ti—O—X having an atom X such as nitrogen, or an anion X, doped in the spaces of the crystal lattice of titanium oxide, and a photocatalyst comprising a titanium compound Ti—O—X having an atom X such as nitrogen, or an anion X, disposed at the grain boundaries of polycrystalline aggregates of titanium oxide crystals (see patent documents 1 to 4).
A further report says that natural gas combustion flames with the temperature of combustion flames maintained in the vicinity of 850° C., for example, by adjusting the flow rates of a natural gas and/or oxygen were struck against a titanium metal to obtain chemically modified titanium oxide n-TiO2—xCx, which absorbed light at 535 nm or less (seen on-patent document 3).
Patent document 1: Japanese Patent Application Laid-Open No. 2001-205103 (claims)
Patent document 2: Japanese Patent Application Laid-Open No. 2001-205094 (claims)
Patent document 3: Japanese Patent Application Laid-Open No. 2002-95976 (claims)
Patent document 4: International Publication 01/10553 brochure (claims)
Non-patent document 1: A. Fujishima et al., J. Electrochem. Soc. Vol. 122, No. 11, p. 1487-1489, November 1975
Non-patent document 2: R. Asahi et al., SCIENCE Vol. 293, Jul. 13, 2001, p. 269-271
Non-patent document 3: Shahed U. M. Khan et al., SCIENCE Vol. 297, Sep. 27, 2002, p. 2243-2245